The Public Switched Telephone Network (PSTN) comprises: a large number of end switching offices serving local subscriber lines and trunks; a smaller plurality of tandem switching offices for providing connections between end offices; a communication network comprising a large plurality of analog and digital communication paths interconnecting the end offices and tandem offices; optional data bases; and a Common Channel Signaling (CCS) overlay signaling network comprising: a number of interconnected Signal Transfer Points (STP)'s for transmitting and switching data messages among the switching offices for managing connection and release of paths of the communication network; and for conveying special service requests. For the purpose of identification in the CCS network, the switching offices of the PSTN are assigned discrete "point codes" which identify a node of the signaling network which is assigned to an office. Similarly, communication paths (trunks) between offices are assigned Circuit Identification Codes (CIC)'s which are used by the offices to establish appropriate connections through their respective communication switching networks. Switching office translations of Circuit Identification Codes identify both the point of termination of the path on the associated communication switching network and characteristics of the path, e.g., analog or digital.
Calls between subscribers are established on the basis of a National Numbering Plan which requires seven or ten digits to define a called subscriber. A seven digit directory number comprises a three digit office code and a four digit line identifier. A ten digit directory number comprises an additional three digit Area Code which precedes the office code. A modem electronic switching system can simultaneously serve a number of such office codes.
There are a number of local situations in which the operations of an existing end office can benefit from the introduction of an additional switching network which shares one or more office codes with an existing switching network. Emerging technology, e.g., introduction of digital facilities to subscribers; population growth in an area served by an end switching office; excessive traffic due to changes in calling patterns handled by an office; replacement of an existing switching network over a period of time; growth of a PBX; and introduction of an alternate service provider are examples of such local situations. When a switching network is added to an existing end office, it is generally desirable that transfer of subscriber lines between an existing switching network and a new switching network be made without changes of directory number.
A number of arrangements for sharing an office code by two switching networks have been proposed in the prior art. One such arrangement is shown in U.S. Pat. No. 5,237,604 which was issued on Aug. 17, 1993. In accordance with that patent, switching networks which share an office code are assigned different point codes and different nodes on the signaling network; and determination of the routing of Initial Address Messages (IAM)'s to the appropriate point code is left to the calling office. An office which is serving an incoming or originating call, determines whether or not the office code of the called number is shared by two switches; and if the office code is shared, a supplementary inquiry is made of a database which is accessible by a number of offices to identify the switching network that is assigned the called directory number. Armed with this information, the calling office can direct the call to the appropriate point code in the signaling network. While this prior art system achieves a desired end result, it: (a) increases the number of nodes on the signaling network; (b) requires independent signaling processors for the networks which share an office code; (c) substantially increases the call processing time of all calling offices; (d) increases traffic on the signaling network; and (e) requires installation of the shared database and updating of the records therein. In summary, this approach requires substantial modification of the overall network to accommodate a shared office code system.
The introduction of U.S. Pat. No. 5,237,604 describes two other prior art systems. One such prior art system routes all calls to an original switching network which passes calls to the added switch over foreign exchange lines; and the other referenced system provides permanent call forwarding of a directory number of a shared office code to a pseudo number of the added switch. Neither of these solutions has proved economically feasible.
Another prior art system is found in U.S. Pat. No. 5,255,315 which was issued on Oct. 19, 1993. In accordance with that patent, switching networks which share an office code are assigned different point codes and different nodes on the signaling network; and determination of the routing of call signaling to the appropriate point code is left to the calling office. If the called office determines that an incoming call is to a subscriber line served by the network receiving the incoming call, i.e., the called switching network, the called switching network: (a) directs the incoming call to the called switching network (itself); and (b) returns an Address Complete "ACCEPT" message to the calling office. However, if an incoming call is to a subscriber line served by an added switching network, the called switching network sends a special "RELEASE" message to the calling office. The release message includes a release "CAUSE" code which indicates that the called directory number is not served by the responding network and identifies the point code of the added network. The calling office generates a new IAM which is directed to the identified point code of the added network. While the system of this patent overcomes some of the problems of the prior art, the requirement for added independent point codes and added independent processors to serve those nodes remain. Additionally, there are messages such as user to user information messages which are not associated with a call having a circuit connection; this arrangement does not serve such messages. Another prior art system is disclosed in U.S. Pat. No. 5,048,081 which employs a "point code converter" to transfer signaling messages to a "new" or an "old" exchange. The two exchanges are served by separate sets of trunks and they, in turn, serve different sets of Directory Numbers. If a signaling message identifies a trunk and a Directory Number served by the "old" exchange, the message is passed to the "old" exchange and the call is handled in the normal way. Similarly, if a signaling message identifies a trunk and a Directory Number served by the "new" exchange, the message is passed to the "new" exchange and the call is handled in the normal way. However, if a signaling message identifies a trunk and a Directory Number which are not served by the same exchange, the exchange "old" or "new" that serves the trunk identified in the signaling message is treated as a "tandem" exchange for the purpose of handling the call. An "added" signaling message is generated which identifies the tandem exchange e.g., the "old" exchange as the source of the message and the other exchange e.g., the "new" exchange as the destination for the signaling message. The "new" exchange is identified by a private point code number known only to the point code converter, the old exchange and the "new" exchange. In the above example, the "old" exchange, acting as a tandem exchange, completes a call connection between the trunk identified in the original message and a tandem trunk to the "new" exchange. The new exchanges acts upon the "added" signaling message and completes a connection between the tandem trunk and the Directory Number which is identified in both the original signaling message and in the "added" signaling message.
While this arrangement provides an interface between a single signal transfer point and two offices that share an office code, calls which involve a trunk and a directory number which are not served by the same network, require a tandem path which includes a connection through the networks of both the "old" and "new" offices and the interconnecting tandem trunk. This solution is costly in that it requires the addition of the tandem trunks between the "old" and "new" exchanges; and it doubles the number of network connections required to handle such tandem connections.